This application is based on and claims priority from Japanese Patent Application No. HEI 4-185157 filed Jul. 13, 1992, the disclosure of which is incorporated by reference herein.
The present invention relates to a telephoto zoom lens system in which part of the front group is used as a focusing lens group.
As the use of an autofocusing mechanism has spread in recent models of cameras, manufacturers of telephoto lens systems have chosen to employ inner focus lenses of a "non-zoom" type in order to reduce the size and weight of the lens group that moves during focusing.
On the other hand, zoom lens systems commonly rely upon the advancing of the front group for achieving focusing and this focusing method has the advantage that it is substantially free of defocusing due to zooming and that it yet experiences comparatively small aberrational variations also due to zooming. This focusing method is also adopted in telephoto zoom lens systems which have a large aperture. However, the moving lenses in telephoto lens systems of this type have such a large aperture that the lens systems are heavier than telephoto lens systems that employ inner focus lenses. As a result, the focusing ring is too heavy to permit swift movement by rotation. Furthermore, accessories such as a filter and a hood are attached to the foremost end of the lens system and, hence, the weight balance with the change in the length of distance over which the moving lenses are displaced will vary so extensively that the handling quality of the lens system during focusing is not satisfactory.
A further problem with the method of focusing by advancing the front lens component is that the spherical aberration is undercorrected if the lens is focused for a near object at the narrow-angle end and that if the closest focusing distance is shortened, an unsharp image of "soft focus" quality will be produced upon full-aperture shooting.
The zoom lens system described in Japanese Patent Publication (Kokoku) No. SHO 63-58324 is designed according to the specifications F 2.8/80-200 mm and achieves focusing by advancing the front group. The lens portion through which an effective bundle of rays will pass has a volume of about 41 cm.sup.3. The layout of lenses in the system is as shown in FIG. 32.
Spherical aberrations that occur when the lens system is focused for an object at infinity with the focal length set at 80 mm are as plotted in FIGS. 33A and 33B; spherical aberrations that occur when the lens system is focused for an object at infinity with the focal length set at 200 mm are as plotted in FIGS. 34A and 34B; spherical aberrations that occur when the lens system is focused for an object 1,500 mm distant with the focal length set at 80 mm are as plotted in FIGS. 35A and 35B; and spherical aberrations that occur when the lens system is focused for an object 1,500 mm distant are as plotted in FIGS. 36A and 36B. Thus, the spherical aberrations that develop when the lens is focused for a near object are greatly undercorrected, particularly at the telephoto end.
Noting these points, the present inventors previously invented a supertelephoto lens system [see Japanese Patent Publication (Kokoku) No. HEI 4-1328] that comprises four lens groups, the first of which was composed of a first sub-group 1a having a positive power, a second sub-group 1b having a negative power and a third sub-group 1c having a positive power. The separation between sub-groups 1a and 1b was sufficient to reduce the diameter of a bundle of rays launching into sub-group 1c, and thereby permitting size reduction of sub-group 1c. Focusing with this system is achieved by moving only sub-group 1c.
The supertelephoto lens system described above successfully provided better handling quality by reducing the weight of the focusing lens group. However, when zooming was effected up to the wide-angle end, such a great vignetting occurred as to cause insufficient aperture efficiency. Hence, the proposal has not been applicable to lens systems that have a half-view angle .omega. of 20.degree. or more at the wide-angle end.